In the hot rolling of the steel sheet bar, since steel sheet bars inclusive of slabs taken out from a heating furnace hitherto been rolled individually, there were various troubles encountered later at a finish rolling step:
a) poor biting of front end of the sheet bar; PA1 b) reduction of back end of the sheet bar; PA1 c) running trouble of front end of the sheet bar on a run-out table; and PA1 d) poor size in front and back ends of the sheet bar. PA1 i) A great amount of power is required in the heating; PA1 ii) The heating time required for obtaining a given joining temperature is long; and PA1 iii) When the heating is carried out at a state that the operation of the heating equipment is stopped, a long loop is required, while when the heating is carried out during the running of the sheet bar, it si required to take a long running distance and hence the equipment length becomes long. PA1 iv) The joining operation is complicated.
As a countermeasure on the above problems, there is proposed a continuous rolling method wherein a back end portion of a preceding sheet bar is successively joined with a front end portion of a succeeding sheet bar at an entry-side transfer line of a hot finish rolling mill and then subjected to finish rolling, and also there are various methods of joining the sheet bars.
For example, Japanese Patent laid open No. 60-244401 discloses an induction heating pressure welding method using a so-called solenoid type coil S (see FIG. 8), and also Japanese Patent laid open No. 61-159285 discloses an electric heating pressure welding method using an electrode roll.
Furthermore, Japanese Patent laid open No. 61-144203 discloses a joining wherein the front end portion of the succeeding sheet bar is butted to the back end portion of the preceding sheet bar and at least side edge regions in widthwise direction at the butted portion are preliminary joined and rolled at a reduction of not less than 20%.
However, in the methods disclosed in Japanese Patent laid open No. 60-244401 and Japanese Patent laid open No. 61-159285, the full back end face of the preceding sheet bar and the full front end face of the succeeding sheet bar are joining faces and the heating is carried out over a whole area of such joining faces, so that there are caused the following problems:
Furthermore, the method disclosed in Japanese Patent laid open No. 61-144203 has problems that a long time is required for obtaining sufficient joining strength, and the problem of the item iii) is still restrained, and also